OLPC XO-1.75 Embedded Controller

Required properties:
- compatible: Should be "olpc,xo1.75-ec".
- cmd-gpios: gpio specifier of the CMD pin

The embedded controller requires the SPI controller driver to signal readiness
to receive a transfer (that is, when TX FIFO contains the response data) by
strobing the ACK pin with the ready signal. See the "ready-gpios" property of the
SSP binding as documented in:
<Documentation/devicetree/bindings/spi/marvell,mmp2-ssp.yaml>.

Example:
	&ssp3 {
		spi-slave;
		ready-gpios = <&gpio 125 GPIO_ACTIVE_HIGH>;

		slave {
			compatible = "olpc,xo1.75-ec";
			spi-cpha;
			cmd-gpios = <&gpio 155 GPIO_ACTIVE_HIGH>;
		};
	};